The present invention relates to processes and devices for adjusting the ignition advance for internal combustion engines supercharged by a turbine-compressor unit.
Conventionally, the ignition advance for a spark ignition engine is controlled by a combination of the action of a centrifugally controlled advance system, which gives an advance as an increasing function of engine speed, and a system controlled by the depression which prevails in the intake pipe of the engine at a point situated slightly upstream of the operator controlled throttle member when the member is in its minimum open position, corresponding to idle running of the engine. As soon as the engine is charged by opening the butterfly valve, the depression pick-up is subjected to the high depression due to the suction of the engine and the system, generally formed by a pneumatic motor, gives a substantial advance to the engine. An advance curve well adapted to the different operating conditions of the engine is then obtained, if the engine is not supercharged.
Attempts to use such a device with an engine supercharged by a turbo-compressor whose turbine receives the exhaust gases of the engine have met with difficulties in obtaining satisfactory matching for all operating conditions of the engine. The difficulties are tied to the fact that the turbo-compressor has no action when the engine is idling or under very low load, so that the engine then receives air under atmospheric pressure, and provides an increasing supercharging pressure from partial load only. Due to supercharging, the ignition advance on full load of the engine must be smaller than that for a non supercharged engine.
Up to now, the approach selected to have satisfactory advance at full load, consists in shifting as a whole the ignition advance curve in the direction of advance decrease. The drawback of that approach is that the amount of advance for partial charges of the engine is too small.
It is an object of the invention to provide an ignition advance curve which is satisfactory under all operating conditions of an engine supercharged by a turbo-compressor.
According to the invention, there is provided a process for control of the ignition advance of a spark ignition engine supercharged by a turbo-compressor, wherein the degree of advance is automatically modified as an increasing function of the speed and as a function of a control pressure which is elaborated from pressures taken from a point of the intake pipe which passes from upstream to downstream of the operator operated throttle member upon opening of said member from its minimum opening position and from a point situated permanently upstream of said throttle member, so that in operation under constant load, the degree of ignition advance increases from idle running in a first speed range, then diminishes in a second range up to maximum speed of the engine.
According to another aspect of the invention, there is provided a device for automatic control of the ignition advance of a spark ignition engine supercharged by a turbo-compressor, comprising a first member for increasing the degree of advance as an increasing function of engine speed and a second control member connected to a point which passes from upstream to downstream of the operator operated throttle member upon opening of said throttle member from its minimum opening position and to a point which is permanently situated upstream of said throttle member so as to cause, for a constant load of the engine, a progressive increase of the degree of advance from idle running, then a decrease up to maximum speed.
The device may be purely mechanical: the second control member may then comprise a movable wall whose movements control the degree of advance, limiting a work chamber, the force exerted on said wall by vacuum prevailing in said chamber biases the movable wall in the direction corresponding to an increase of the degree of advance against the action of a first spring, whereas the force exerted on said wall by overatmospheric pressure in said work chamber biases the wall in a direction corresponding to a decrease of the degree of advance, against the action of a second spring.
The device may also be implemented by using electronic means; this solution is of particular advantage if the engine is fitted with a digital or analog computer which also fulfils other functions, such as metering the amount of fuel admitted to the engine.
The invention will be better understood from the following description of embodiments of the invention, given by way of examples. The description refers to the accompanying drawings.